1. Technical Field
This invention relates to polymers. More particularly, it relates to processes for the preparation of polymers using metallocene catalysts.
2. Background of the Art
Over the last several years it has been shown by a number of researchers that use of metallocene catalysts offers improvements in polymer properties when compared with production of similar polymers using Ziegler-Natta catalysts. For example, metallocene catalyzed polypropylene materials typically have lower xylene solubles than Ziegler-Natta catalyzed polymers. Such polyolefins typically have narrow molecular weight distributions. For example, films produced using metallocene catalyzed polyethylene resins may be used to produce films with increased clarity as compared to otherwise similar films produced with Ziegler-Natta resins. At least some of the improvements are attributable to the increased control of stereochemistry, molecular weight distribution, and comonomer incorporation obtainable with the metallocenes. This control enables tailoring of the final polymer and may offer improvements in areas as diverse as optical performance, strength performance, and the like.
Frequently cocatalysts are employed with metallocene catalysts. Frequent and effective choices for metallocene production runs are, for example, tri-isobutylaluminum (“TIBAl”) and tri-ethylaluminum (“TEAl”). Such catalyst systems may be employed in conventional polymerization equipment such as, for example, loop reactors.
Unfortunately, TIBAl, for example, tends to decompose at elevated temperatures to form isobutylene. Isobutylene is a molecule that is very similar in molecular weight and boiling point to the isobutane diluent often employed in polymerization reaction mixtures. The result of this similarity is that a high steady-state isobutylene level may result during polymerization because the isobutylene is not readily separated from the isobutane diluent in the recycle purification system. In some cases, these materials can lower catalyst mileage. Additionally, the presence of alkenes not readily removed from the diluent can lead to production issues as this results diluent contaminated with alkene byproduct, in the case of TIBAl isobutylene, being sent to portions of the process where olefins can detrimentally affect the product process, for example, the catalyst feed and flush systems as well as various mechanical seal flushes.